respiration is carried on in large tribes of creatures, whose body is small in size and delicate in texture, and the functions of whose economy are performed with energy (fig. XV.). [Illustration: Fig. XV. The Achilles Butterfly of South America (_Papilio Achilles_), showing the tracheæ on the upper and under side of the wings.] But this contrivance will not do when the animal is of large magnitude; when its body is divided into numerous compartments; when these compartments extend far beneath the external surface; when important organs are placed in deeply-seated cavities; and when the substances that compose the organs are dense, hard, thick, and convoluted. To construct air-tubes of the requisite diameter and length, always open, always in a condition to permit the ingress and egress of an adequate current of air to and from the remotest nook and corner of a body such as this, would be difficult, if not impossible. At all events, it is easier, in such a case, to carry the food to the air, than the air to the food. But, for the accomplishment of this purpose, what is necessary? An organ for containing food; an organ for containing air; vessels to carry food to and from the receptacle of the aliment; vessels to carry air to and from the receptacle of the air; expedients to expose a stream of food to a current of air; and, finally, tubes to carry out to the system the product of this complicated operation. Accordingly, a reservoir of food and a reservoir of air; an apparatus by which both are conveyed to their respective receptacles; and an apparatus by which both are brought into contact sufficiently close to admit of their mutual action, are all combined in the lung of the animal, and in the mechanism by which its movements are effected. The object is accomplished, but the apparatus by which it is effected is as complex in structure as it is efficient in action; the result simple; the means by which the result is secured, highly complicated.